Evaluating tidal impact on ground source heat exchanger performance under fluctuating groundwater levels

• Published in: Geothermics 2024-07, Vol.121, p.103050, Article 103050
• Main Authors: Fujii, Hikari, Mohammadzadeh Bina, Saeid, Sato, Nanami, Ishihara, Takeshi
• Format: Article
• Language: English
• Subjects: FEFLOW; groundwater; GSHP; HDD; Tidal; TRT
• ISSN: 0375-6505; 1879-3576
• DOI: 10.1016/j.geothermics.2024.103050

•The impact of tidal fluctuations on groundwater levels was assessed.•The feasibility of using horizontal directional GHE in coastal areas was investigated.•The optimal installation depth for HDD-GHE in coastal regions was determined.•The most significant tidal-influenced layer is the suitable area for HDD-GHEs.•TRTs were conducted on a vertical well located 150 m from the coastline. Coastal regions undergo dynamic changes due to tidal forces, posing distinct challenges for groundwater management and utilization of ground source heat pump (GSHP) systems. This study examines the impact of tidal fluctuations on groundwater levels and the performance of ground heat exchangers (GHEs), specifically focusing on the suitability of horizontal directional drilling (HDD) technology in high-tidal zones. Itoman City in Okinawa Prefecture, Southern Japan, serves as an ideal location for this study due to its complex coastal hydrogeological conditions and increasing interest in renewable energy solutions. Employing a comprehensive approach involving field experiments, FEFLOW software simulations, and model validation, we aim to elucidate the intricate relationship between tidal dynamics, groundwater behavior, and GHE performance. Furthermore, our investigation assesses the effectiveness of HDD-GHEs installed in highly tidal-affected zones to determine the optimal installation depth for this type of GHE in coastal regions. According to our research, the layer at a depth of 30 m is most affected by tidal influence and fluctuations in groundwater levels in coastal regions. This layer is the most suitable for installing horizontal HDD-GHEs, with an average heat exchange rate of about 89 W/m. This rate is approximately four and two times higher than the depths above and below this layer, respectively, due to groundwater saturation and flow in the formation. Furthermore, we noticed that the heat exchange rate increases from the top to the bottom of this layer, but the extent of this increase diminishes as the depth increases. This is because deeper depths are less affected by tidal fluctuations in groundwater levels. Step 1: evaluation of water table fluctuation in the coastal regions by tides during high tide level (HTL) and low tide level (LTL). Installing of a horizontal ground heat exchanger (HDD-GHE) in the highly affected layer. [Display omitted]